Multi-antenna systems are widely considered to be the most viable way for significantly increasing the bandwidth efficiency of wireless data transmission systems. In MIMO (multiple input multiple output) systems, multiple antennas are deployed both at the transmitter and the receiver. In MISO (multiple input single output) systems, the receiver has only one antenna, and the multiple transmit antennas are used for transmit diversity. In SIMO (single input multiple output) systems, the transmitter has a single antenna, and multiple antennas are used at the receiver.
Given multiple antennas, the spatial dimension of the channel can be exploited to improve the performance of the wireless link. The performance is often measured as the average bit rate (bit/s) the wireless link can provide, or as the average bit error rate (BER), depending on the application.
Given a multi-antenna channel, a duplex method, and a transmission bandwidth, the multiple-antenna system can be categorized as narrowband or wideband (i.e., the channel is flat or frequency selective fading within the system bandwidth), and possessing either full, partial or no channel state information (CSI).
Multiple antennas provide enhanced performance. Studies show that the multiple antennas can be used to provide spatial diversity, and/or can increase the information-theoretic capacity (data rate), see, e.g., Vaughn and Anderson, Channels, propagation, and antennas for mobile communications, IEE Press, 2003, Telatar et al., “Capacity of Multi-Antenna Gaussian Channels,” European Trans. on Telecomm., Vol. 10, No. 6, pp. 585-596, November-December 1999, Winters, “On the Capacity of Radio Communication Systems with Diversity in Rayleigh Fading Environments,” IEEE J. Selected Areas Comm., 1987, and Tarokh et al., “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,” IEEE Trans. Inform. Theory, Vol. 44, pp. 744-765, March 1999.
However, as a disadvantage, operation with multiple RF chains increases complexity and cost. An antenna selection technique can be used to determine an optimal subset of antennas. This still yields improved performance while reducing the number of required RF chains, see Molisch et al., “Capacity of MIMO Systems with Antenna Selection,” Proc. IEEE Intl. Comm. Conf., pp. 570-574, 2001, and Gore et al., “MIMO Antenna Subset Selection with Space-Time Coding,” IEEE Trans. Signal Processing, Vol. 50, No. 10, pp. 2580-2588, October 2002.
Most prior art antenna selection techniques have focused only on selecting a subset of antennas before down-conversion and processing in baseband. This (spatial) antenna selection techniques work reasonably well under some circumstances, namely in the case that (i) the number of selected antennas is only slightly smaller than the number of available elements, and (ii) the MIMO channel is spatially uncorrelated.
However, in reality, correlated scattering at both the transmitter and receiver antenna arrays is more usual. Due to the directional transmission in the wireless environment, the signal waveforms can be highly correlated depending on the departure and arriving angles at the antennas.
The prior art antenna selection techniques that operate only in the spatial domain can have a significant degradation in performance when dealing with this correlation. Furthermore, even for weak correlations, the prior art antennas also show a significant performance loss when the number of selected antennas is considerably smaller than the number of available antennas.
Therefore, it is desired to provide an RF signal processing technique that overcomes the problems of the prior art.